NEW CONSTRAINTS ON THE DEPOSITIONAL AGE OF PALEOPROTEROZOIC QUARTZITE EXPOSURES IN PHANTOM CANYON, CANON CITY, COLORADO

A thin (~10 meters) panel of compositionally mature Paleoproterozoic quartzite is exposed along Phantom Canyon approximately 13 km northeast of Canon City, Colorado. The unit trends northeast and dips moderately southeast. The quartzite is in contact with a coarse-grained granite to the north that has been mapped as part of the ca. 1665 Ma Cripple Creek batholith (Bickford et al., 1989). The quartzite is in contact with migmatitic gneiss to the south interpreted to be in thrust or fault contact, but the contact relationships between the quartzite and granite to the north were previously not well established. New field mapping and outcrop observation suggest that the quartzite is exposed in the eroded remains of a tight, overturned syncline that might represent the roots of a larger fold. We also report a previously unmapped basal conglomerate that is exposed stragraphically beneath the quartzite adjacent to the contact with the Cripple Creek granite. Beneath the basal conglomerate, granite was observed to have a well developed foliation defined by potassium feldspar that grades upward into a phyllitic unit with a biotite foliation, limited feldspars, and some quartz pebbles. This phyllitic layer grades into the Phantom Canyon basal conglomerate, which is dominated by folded cm-scale quartz clasts. We interpret the transition from granite to phyllite, basal conglomerate, and quartzite to represent an unconformable depositional contact that was subsequently sheared, consistent with relationships described in quartzite exposures along Blue Ridge approximately 25 km to the west (Jones et al., 2009). Detrital zircon from quartzite exposed along Phantom Canyon yielded a minimum zircon age of 1701±5 Ma, and this age has been interpreted as the maximum depositional age for this quartzite and other likely correlative quartzites that are exposed throughout the surrounding region. Continued research will focus on obtaining an age of the granite exposed adjacent to the quartzite in Phantom Canyon, and more extensive mapping will test the correlation of the locally exposed granite with granite exposed at the dated locality of Bickford et al. (1989). Ultimately we hope that our results will help to better constrain the depositional age of quartzites locally, which will provide new insights into the tectonic evolution of southern Laurentia during the Paleoproterozoic.